Generally, in an OFDM communications system, a channel comprising a large number of sub-carriers over a frequency range is used to carry data. Frequencies within the channel may be referred to as in-band frequencies, and frequencies outside of the channel may be referred to as out-of-band frequencies. For example, if an OFDM communications system supports a plurality of channels, with a transmission occurring over one channel, then transmissions over the one channel are in-band transmissions with respect to the one channel, while transmissions occurring outside of a frequency range of the one channel are out-of-band transmissions with respect to the one channel.
Potentially, the OFDM communications system uses a number of different channels to allow communications by multiple communications devices operating in the OFDM communications system. The sub-carriers within a channel may be closely spaced and orthogonal to one another. The information (data and/or pilots) to be carried may be divided into multiple information streams with one information stream per sub-carrier. Each sub-carrier may be modulated with a conventional modulation scheme, such as quadrature amplitude modulation or phase shift keying.
In addition to the sub-carriers, a channel may include guard bands placed on the upper and lower ends of the channel frequency range, surrounding the frequency range of the sub-carriers. The guard bands may be used to help prevent signals in the channel from leaking out of the channel and causing interference to other communications devices in the OFDM communications system as well as electronic devices not in the OFDM communications system. For example, with the OFDM wireless communications system operating in a channel (or channels) with a frequency band adjacent to licensed spectrum, by regulation, the OFDM wireless communications system must not cause interference to electronic devices that are using the licensed spectrum. Additionally, the communications devices communicating over the channel (channels) should not cause interference to communications devices communicating over channels adjacent to the channel.
However, even with the use of guard bands, signals on the sub-carriers in the channel will leak through into adjacent channels or frequencies and may raise the signal level on out-of-band frequencies. This may negatively impact the performance of electronic devices operating in the out-of-band frequencies.
In co-assigned U.S. patent applications entitled “Ultra Wideband Interference Cancellation for Orthogonal Frequency Division Multiplex Transmitters by Protection-Edge Tones,” U.S. patent application Ser. No. 11/173,840, filed Jul. 1, 2005, and “Narrow Band Interference Cancellation Technique for OFDM Transmitters,” U.S. patent application Ser. No. 11/356,780, filed Feb. 17, 2006, which are incorporated herein by reference, a focus is placed on tone-nulling applications by controlling the interference level affecting narrowband communications systems, which usually has a right to use the band. The techniques proposed may not have application in sidelobe suppression since an optimization range may be set only within the OFDM signal spectrum, and therefore, sidelobes may not be effectively suppressed.
In a technical paper by S. Brandes, I. Cosovic, and M. Schenell, entitled “Sidelobe Suppression in OFDM Systems by Insertion of Cancellation Carriers,” IEEE Vehicular Technology Conference (VTC-2005-Fall), September 2005, which is incorporated herein by reference, a technique for sidelobe suppression is presented. However, the technique ignores the effects of a transmitter's many baseband operations, including cyclic prefix and transmit time-windowing. Furthermore, a simple sinc function is used in calculating the interference assuming a rectangular pulse shaping for OFDM symbol and no cyclic prefix applied. Therefore, the technique has limited application in OFDM communications systems having cyclic prefix and transmit time-windowing.
Therefore, what is needed is a system and method for sidelobe suppression in OFDM communications systems that is applicable to OFDM communications systems wherein transmitters perform a number of baseband operations on the information to be transmitted, including adding a cyclic prefix (or cyclic suffix) and using transmit time-windowing (or time-domain filtering).